In recent years, to simultaneously achieve lower costs and higher efficiency of a photoelectric conversion device, thin-film photoelectric conversion devices having almost no problem from the standpoint of resources have attracted attention, and development thereof has been tried vigorously. Application of the thin-film photoelectric conversion devices to various uses such as solar batteries, optical sensors, displays and others have been expected. An amorphous silicon photoelectric conversion device as one type of the thin-film photoelectric conversion devices can be formed on a low-temperature, large-area substrate such as glass substrate or stainless steel substrate, with which cost reduction is expected.
A thin-film photoelectric conversion device generally includes a first electrode, one or more semiconductor thin-film photoelectric conversion units, and a second electrode, which are successively stacked on a substrate having an insulative surface. Each thin-film photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.
The i-type layer, which is a substantially intrinsic semiconductor layer, occupies the most part of thickness of the thin-film photoelectric conversion unit. Photoelectric conversion occurs primarily in this i-type layer. Thus, a thicker i-type photoelectric conversion layer is preferable from the standpoint of light absorption, though an unnecessarily thick layer leads to increase of both costs and time for deposition thereof.
On the other hand, the p-type and n-type conductive layers serve to generate diffusion potential in the photoelectric conversion unit. The magnitude of the diffusion potential influences the value of open-circuit voltage that is one of the critical characteristics of the thin-film photoelectric conversion device. These conductive layers, however, are inactive layers not contributing to photoelectric conversion. Light absorbed by the impurities introduced into the conductive layers becomes loss without contributing to generation of electric power. As such, it is preferable that the p-type and n-type conductive layers are as thin as possible within a range ensuring generation of sufficient diffusion potential.
A photoelectric conversion unit or a thin-film solar battery is called an amorphous photoelectric conversion unit or an amorphous thin-film solar battery in the case that it includes an amorphous i-type photoelectric conversion layer occupying its main part whether its p-type and n-type conductive layers are amorphous or crystalline, and is called a crystalline photoelectric conversion unit or a crystalline thin-film solar battery in the case that it includes a crystalline i-type layer.
In general, in a semiconductor used for the photoelectric conversion layer, the light absorption coefficient decreases as the light wavelength increases. In particular, when the photoelectric conversion material is in a state of a thin film, sufficient light absorption is not expected in the wavelength region of small absorption coefficient, and thus the photoelectric conversion amount is restricted depending on the thickness of the photoelectric conversion layer. Therefore, measures have been taken to generate a large amount of photocurrent, by forming a light scattering structure for preventing light having come into the photoelectric conversion device from easily escaping to the outside, to thereby increase the substantial optical path length and cause sufficient absorption. For example, when the light is incident on a transparent substrate side, a textured transparent conductive film having fine unevenness is used as a light incident side electrode.
Further, as a way of improving the conversion efficiency of the thin-film photoelectric conversion device, it is known to form a stacked-layer type thin-film photoelectric conversion device having at least two photoelectric conversion units stacked one another. In such a way, a front photoelectric conversion unit including a photoelectric conversion layer having a large band gap is arranged on a light incident side of the photoelectric conversion device, and back photoelectric conversion units each including a photoelectric conversion layer (of, e.g., Si—Ge alloy) having a smaller band gap in turn are successively arranged on the back of the front unit so as to enable photoelectric conversion over a wide wavelength range of the incident light, to thereby improve the conversion efficiency of the entire device. Of the stacked-layer type photoelectric conversion devices, one including both the amorphous and crystalline photoelectric conversion units is called a hybrid type photoelectric conversion device. In the hybrid type photoelectric conversion device, the wavelength of light photoelectrically convertible with the amorphous silicon is about 800 nm on the longer wavelength side, whereas the light of longer wavelength of up to about 1100 nm can be photoelectrically converted by the crystalline silicon, so that effective photoelectric conversion becomes possible over a wider wavelength range of the incident light.
In the stacked-layer type photoelectric conversion device, the photoelectric conversion units are connected in series. The shorted-circuit current density (Jsc) of the photoelectric conversion device is restricted by the smallest one of current values generated by the photoelectric conversion units therein. Thus, it is preferable that the current values of the photoelectric conversion units are as even as possible. Further, improvement of conversion efficiency is expected with the greater absolute value of the current. In the stacked-layer type photoelectric conversion device, a conductive intermediate reflective layer having both light transmitting and reflecting properties may be interposed between the photoelectric conversion units. In this case, light having reached the intermediate reflective layer is partially reflected, and thus it is possible to increase the light absorption amount and then increase the current generated within the front photoelectric conversion unit located closer to the light incident side than the intermediate reflective layer. This means that the effective thickness of the front photoelectric conversion unit is apparently increased.
For example, in the case that an intermediate reflective layer is inserted in a hybrid type photoelectric conversion device formed of a front amorphous silicon photoelectric conversion unit and a back crystalline silicon photoelectric conversion unit, the current generated in the front photoelectric conversion unit can be increased without increasing the thickness of the amorphous silicon photoelectric conversion layer. Further, when the intermediate reflective layer is included, the thickness of the amorphous silicon photoelectric conversion layer required to obtain the same current value can be decreased as compared to the case not including the intermediate layer. As such, it is possible to restrict deterioration in properties of the amorphous silicon photoelectric conversion unit due to the optical deterioration (Sraebler-Wronsky effect) that becomes considerable in accordance with increase in thickness of the amorphous silicon layer.
A conventional intermediate reflective layer is often formed of TCO (transparent conductive oxide) such as polycrystalline ITO (indium tin oxide) or ZnO, particularly of ZnO. The ZnO intermediate reflective layer, however, is formed by sputtering or spraying, which requires a film deposition apparatus besides the plasma CVD (chemical vapor deposition) apparatus generally used for formation of semiconductor films. This increases the equipment costs and the production tact time. Further, when sputtering is employed for formation of the ZnO layer, there is a possibility that the underlying semiconductor film suffers degradation in its properties due to damages caused by sputtering.
Moreover, it is necessary to form a good ohmic contact at the interface between the TCO intermediate reflective layer and the semiconductor layer so as to suppress an adverse effect on the series resistance of the stacked-layer type photoelectric conversion device. However, it is generally known that it is not easy to form an ohmic contact at an interface between a ZnO layer and an amorphous silicon layer or a crystalline silicon layer. More specifically, if the dark conductivity of the ZnO intermediate reflective layer is lower than 1.0×102 S/cm, the intermediate reflective layer cannot form a good ohmic contact with the front photoelectric conversion unit or with the back photoelectric conversion unit, leading to increase of the contact resistance, which will in turn decrease the fill factor (FF) of the stacked-layer type photoelectric conversion device. On the contrary, if the dark conductivity of the ZnO layer is greater than 1.0×103 S/cm, the light transmittance will decrease, and then the shorted-circuit current density (Jsc) of the stacked-layer type photoelectric conversion device will decrease. As such, it is necessary to set the dark conductivity of the TCO layer at a relatively high level in a range from 1.0×102 S/cm to 1.0×103 S/cm by impurity doping or by adjusting the degree of oxidation.
A large-area thin-film photoelectric conversion device is generally formed as an integrated type thin-film photoelectric conversion module. The integrated type thin-film photoelectric conversion module has a structure in which a plurality of photoelectric conversion cells are separated from each other to have their respective small areas and are electrically connected in series on a glass substrate. Normally, each photoelectric conversion cell is formed by successively depositing and patterning, on the glass substrate, a transparent electrode layer, one or more thin-film semiconductor photoelectric conversion unit layers, and a back electrode layer.
FIG. 30 is a schematic cross sectional view of an example of a conventional integrated type thin-film photoelectric conversion module having a plurality of stacked-layer photoelectric conversion cells connected in series, provided with no intermediate reflective layer. Incidentally, throughout the drawings, the same reference characters denote the same or corresponding portions. A photoelectric conversion module 101 has a structure in which a transparent electrode layer 103, a front amorphous silicon photoelectric conversion unit layer 104a, a back crystalline silicon photoelectric conversion unit layer 104b, and a back electrode layer 106 successively stacked on a glass substrate 102.
Integrated type thin-film photoelectric conversion module 101 is provided with first and second isolation grooves 121, 122 for electrically isolating photoelectric conversion cells 110 from each other, and a connection groove 123 for electrically connecting the cells in series. First and second isolation grooves 121, 122 and connection groove 123 are parallel to each other, and extend in the direction perpendicular to the paper plane of FIG. 30. That is, first isolation groove 121 separates transparent electrode layer 103 into a plurality of regions, corresponding to the respective photoelectric conversion cells 110. Similarly, second isolation groove 122 separates front photoelectric conversion unit layer 104a, back photoelectric conversion unit layer 104b, and back electrode layer 106 into a plurality of regions, corresponding to the respective photoelectric conversion cells 110.
Connection groove 123 provided between first isolation groove 121 and second isolation groove 122 penetrates through front and back photoelectric conversion unit layers 104a and 104b. Connection groove 123 is filled with the same metal material as that of back electrode layer 106, and electrically connects in series back electrode 106 of one photoelectric conversion cell 110 to transparent electrode 103 of the neighboring photoelectric conversion cell 110.
An integrated type thin-film photoelectric conversion module of FIG. 31 differs from the module of FIG. 30 only in that a TCO intermediate reflective layer 105 is inserted between front and back photoelectric conversion unit layers 104a and 104b. In the photoelectric conversion module of FIG. 31, connection groove 123 penetrates through front photoelectric conversion unit layer 104a, TCO intermediate reflective layer 105, and back photoelectric conversion unit layer 104b, and is filled with the same metal material as that of back electrode layer 106. That is, the metal material filled in connection groove 123 comes into contact with TCO intermediate reflective layer 105.
TCO intermediate reflective layer 105 has high dark conductivity in the range of 1.0×102 S/cm to 1.0×103 S/cm as described above, and thus electric current can easily flow through TCO layer 105 in a direction parallel to substrate 102. Therefore, back photoelectric conversion unit 104b is short-circuited due to the current path through TCO intermediate reflective layer 105, connection groove 123 and back electrode layer 106, causing a large leakage current. As a result, in the photoelectric conversion module of FIG. 31, the electric power generated at back photoelectric conversion unit 104b can hardly be taken out.